Chemistry explained
What Is a Solvent?
Solvents dissolve, thin, clean and carry. The choice between them comes down to polarity, volatility and safety — and getting it wrong is expensive. Here is the working guide.
In short
A solvent is a substance, usually a liquid, that dissolves another substance to form a solution without chemically changing it. The solvent is the component present in the larger amount; the substance being dissolved is the solute. Water is the most common solvent on earth, but industry runs largely on organic solvents such as acetone, isopropyl alcohol, toluene and ethyl acetate.
The one rule that governs everything: like dissolves like
Whether a solvent will dissolve a given material comes down to how well their intermolecular forces match. Polar solvents dissolve polar solutes; non-polar solvents dissolve non-polar solutes. Salt, an ionic solid, dissolves readily in water because water molecules are strongly polar and can surround and stabilise the separated ions. Candle wax, a long non-polar hydrocarbon, does not dissolve in water at all, but dissolves easily in hexane, which is equally non-polar.
This is why a degreaser that works on machine oil may be useless on a water-based adhesive, and why cleaning a printed circuit board of flux residue calls for isopropyl alcohol rather than water. It is also why blends are so common in industry: a mixed solvent lets a formulator hit a polarity between the two pure components and dissolve materials that neither would handle alone.
The three classes of solvent
Chemists sort organic solvents into three groups based on polarity and whether the molecule has a hydrogen atom it can donate.
| Class | Examples | Dissolves | Typical use |
|---|---|---|---|
| Polar protic | Water, methanol, ethanol, isopropyl alcohol | Salts, polar organics, sugars | Cleaning, disinfection, extraction |
| Polar aprotic | Acetone, ethyl acetate, acetonitrile, DMF | Wide range of organics; not salts | Degreasing, coatings, synthesis |
| Non-polar | Hexane, heptane, toluene, xylene | Oils, fats, waxes, rubbers, resins | Paint thinning, extraction, degreasing |
Polar protic
Polar, and carrying an O–H or N–H bond, so they can donate hydrogen bonds. Water, methanol, ethanol and isopropyl alcohol belong here. They dissolve salts and polar organics well, and they solvate both cations and anions. In organic synthesis they favour SN1 reaction pathways.
Polar aprotic
Polar, but with no acidic hydrogen to donate. Acetone, ethyl acetate, acetonitrile, DMF and DMSO. These dissolve a wide range of organics and are the standard choice where a reaction needs a polar environment but must not be quenched by a proton source. Acetone is the highest-volume member and the most common industrial cleaning solvent.
Non-polar
Hexane, heptane, toluene, xylene, white spirit. These dissolve oils, fats, waxes, rubbers and most resins, and they are the backbone of paint thinners, extraction processes and degreasing. They will not dissolve salts at all.
The properties that decide the choice
Polarity narrows the field. These five properties usually pick the winner.
Boiling point and evaporation rate
A fast-evaporating solvent such as acetone leaves a surface dry in seconds, which is ideal for cleaning and for lacquers that must set quickly. It is a liability in a coating that needs time to level, where flash-off that is too fast causes orange peel and blush. Slow solvents give flow and levelling but extend drying and oven time.
Flash point
The lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture. It drives everything about how the solvent is stored, transported and used. Acetone’s flash point is around −20 °C and isopropyl alcohol’s is 12 °C, so both are flammable at any ambient temperature and both ship as Class 3 dangerous goods. A solvent with a flash point above 60 °C is generally not classed as a flammable liquid for transport, which is a substantial cost difference.
Toxicity and exposure limits
Occupational exposure limits vary by two orders of magnitude across common solvents. Acetone carries an OSHA PEL of 1000 ppm; benzene, which it might otherwise resemble in solvency, is regulated at 1 ppm and is a known human carcinogen. Substitution towards a less toxic solvent with similar performance is normally the cheapest control available.
Material compatibility
Solvents attack polymers. Acetone dissolves polystyrene, PVC and many acrylics outright, and will craze polycarbonate. Before a solvent goes into a process, check every seal, gasket, hose and container it will contact. This is one of the most common and most expensive oversights in solvent selection.
Residue and purity
For electronics and pharmaceutical use, what the solvent leaves behind matters more than what it dissolves. A technical grade solvent with 0.5 per cent non-volatile residue is fine for degreasing a casting and unusable for cleaning a wafer.
Industrial uses
- Cleaning and degreasing — removing oils, greases, fluxes and adhesives from metal, glass and electronics
- Paints, inks and coatings — dissolving the resin, controlling viscosity for application, and setting the drying profile
- Adhesives — carrying the polymer to the substrate and evaporating away
- Extraction — pulling target compounds out of natural material, from vegetable oil to pharmaceutical intermediates
- Chemical synthesis — providing the reaction medium and controlling reaction pathway and rate
- Pharmaceutical processing — crystallisation, granulation and purification, under ICH residual solvent limits
Handling solvents safely
Most industrial solvents share three hazards: they are flammable, they depress the central nervous system when inhaled, and they defat skin on repeated contact. The controls follow directly.
- Eliminate ignition sources and control static — ground and bond containers during transfer, because a flowing solvent generates charge
- Ventilate. Solvent vapour is heavier than air and collects in pits, sumps and low corners where it can reach explosive concentration
- Choose gloves by breakthrough data, not by habit. Nitrile is permeated by acetone in minutes; butyl rubber holds
- Store in a fireproof area with no drain access, in closed containers, away from oxidisers
- Check whether the solvent forms peroxides. Ethers and secondary alcohols such as isopropanol do, and peroxides concentrate dangerously on distillation
Detailed hazard data for the solvents we supply is on our acetone and isopropyl alcohol reference pages.
Frequently asked questions
What is the difference between a solvent and a solute?
The solvent is the component present in the greater amount and does the dissolving; the solute is what gets dissolved. In brine, water is the solvent and salt is the solute. The distinction is about proportion rather than any intrinsic property — in a mixture of two miscible liquids, whichever is in excess is conventionally called the solvent.
Is water a solvent?
Yes, and the most important one. Water is a strongly polar protic solvent and dissolves a wider range of ionic and polar substances than any other single liquid, which is why it is called the universal solvent. The name overstates it: water dissolves almost nothing non-polar, which is exactly why organic solvents exist.
Which solvent is best for degreasing?
It depends on the soil and the substrate. Acetone is fast, cheap and dissolves a very wide range of organic residues, which makes it the default for general metal degreasing, but it attacks many plastics. Isopropyl alcohol is gentler on polymers and leaves less residue, making it standard for electronics. Non-polar hydrocarbons are better for heavy mineral oil and grease.
What does flash point tell you about a solvent?
It is the lowest temperature at which the liquid produces enough vapour to form an ignitable mixture with air. It determines the transport classification, the storage requirements and the ignition controls needed in use. A solvent whose flash point is below ambient temperature is always giving off ignitable vapour whenever a container is open.
Are green solvents a real alternative?
In many applications, yes. Ethyl lactate, 2-methyltetrahydrofuran, limonene and supercritical carbon dioxide all replace conventional solvents in specific processes with a better toxicity or biodegradability profile. They are usually more expensive and rarely a drop-in substitute, so the switch generally requires reformulation rather than a straight swap.
Why do solvents attack plastics?
Polymer chains are held together by the same kind of intermolecular forces that govern dissolution. A solvent of matching polarity can push between the chains, swelling the polymer, softening it and eventually dissolving it. Under stress this shows up as environmental stress cracking, where a part fails at a load it would otherwise carry easily.
Products referenced on this page
Supplied in bulk from Chinese manufacturing plants with a batch certificate of analysis and the manufacturer’s safety data sheet against every consignment.